The invention relates to techniques for detecting organic vapors, aerosols and volatile substances, e.g., as produced in a fire.
Organic vapors and aerosols from fires can be detected by various known methods, with different methods being used in commercial smoke detectors and in laboratory equipment. Among the former are so-called ionization smoke detectors in which the air in an ionization chamber is ionized by a radioactive source, and an electric field is applied for measurement of a resulting ionic current. When an aerosol is present in the ionization chamber, the ionic current is reduced. In other commercial smoke detectors, and in measurement instruments, optical methods are used for detecting aerosols and vapors, e.g., by measuring optical transmission, spectral absorption or light scattered by aerosols. Used to a lesser extent is detection by surface ionization in which molecules are ionized thermally at a heated surface, and the ions are measured as an ionic current in an applied electric field.
In selecting a detection method, it is often desirable that different vapors and aerosols be detected with equal sensitivity. The detection of aerosols by ionization smoke detectors is reliable in this respect, as most combustion vapors and aerosols cause a reduction in the ionic current within a short time span. However, in commercial applications the need for a radioactive source to produce the ionic current causes difficulties, e.g. in the disposal of radioactive waste in the manufacture and maintenance of such detectors.
Optical detection by measuring light scattered by aerosols has different sensitivities for different combustion aerosols, as the degree of light scattering depends on the type of aerosol. Light-colored aerosols scatter incident light to a considerable degree, and more so than dark-colored aerosols.
Optical transmission measurements are superior with respect to uniformity of detection of different types of aerosols, but are difficult to carry out especially at small concentrations. The detection of small concentrations requires either a long measurement distance, or else a measurement with very high sensitivity. These measurements of transmission have been found to be difficult and of limited applicability.
Absorption measurement requires special light sources, filters and sensors which tend to be costly. Moreover, for the detection of different types of aerosols it is necessary to replace certain device components, so that this method is limited to laboratory equipment.
In the detection of vapors and aerosols by surface ionization, molecules to be detected condense at a conductive surface. To some extent, molecules are adsorbed at the surface. The molecules are ionized by heating the surface to several hundred degrees Celsius, and positive and negative ions are emitted from the surface. For measurement, emitted ions are collected by a counter-electrode which faces the surface. Depending on whether the charge of the ions to be detected is positive or negative, an opposite negative or positive potential is applied to the electrode. By measuring the resulting ionic current, ionizable molecules in the vicinity of the measurement device are detected.
Effective detection of aerosols by surface ionization depends primarily on the degree of ionizability of the molecules. The degree of ionizability depends on the temperature of the ionizing surface and on the particle ionization potential (in the case of positive ions) or the electron affinity (in the case of negative ions) as compared with the emission work function of the surface material. Positive ions are formed best when the ionization potential is low, and negative ions when the electron affinity is high.
Substances whose vapors and aerosols can be detected by surface ionization, i.e., which can be ionized at a conductive surface, include amines, hydrazines and nitrogen-containing compounds, e.g., triethylamine, trimethylamine, pentylamine, butylamine, propylamine, amino acids, acetic acids, phenols, anilines and ammonia. In particular, aerosols and vapors produced by burning pulp materials can be detected by this method.
A device for the detection of smoke particles is described in U.S. Pat. No. 4,176,311. The device has a wire of tungsten or platinum, and a cylindrical or semi-cylindrical counter-electrode. At the wire, smoke particles are ionized thermally, as wire temperature is at a constant 500.degree. C. due to a steady electric current in the wire. There results a measurable ionic current to the counter-electrode. In commercial use especially, the limited life of the heating wire is disadvantageous. Also, as the wire becomes contaminated, the sensitivity of the device decreases over time. Moreover, for continuous heating of the wire, power consumption is relatively high.